Effect of online rapid cooling on microstructure and mechanical properties of friction stir welded medium carbon steel

Motivated by the morphological change in the fine ferrite grains and cementite particles from post-annealing, and its consequences of the loss of strength and ductility, friction stir welds of medium carbon steel were obtained under rapid cooling via liquid CO2 at a temperature below the A1 transformation temperature. Equiaxed ultrafine ferrite grains to the average grain size of less than 1 μm are achieved under the rapid cooling due to much faster elimination of heat when compared with the welds obtained under natural cooling. The significant effect of the rapid cooling is observed in the transition regions between the stir zone and base metal. The grains are severely deformed and elongated under rapid cooling, while equiaxed grains with some grains showing coarsening due to the post annealing are the dominant features under natural cooling. In addition, a decrease in the size of the cementite particles is also observed under the rapid cooling. These changes are responsible for the improvement in the mechanical properties of the obtained joint under rapid cooling. The fine dispersion of the cementite particles under rapid cooling leads to an improved work hardening rate which helps to counter the negative effect of the grain refinement on the ductility.